May 2015 - Beginners - Haskell.org

how can I make this work
by Roelof Wobben 13 May '15

13 May '15

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AMQP and nested exception handlers
by Alex 13 May '15

13 May '15

Hi: I am writing a small application which receives HTTP requests, translates them to JSON, and queues the requests using RabbitMQ. I am using exceptions to handle extreme cases, such as when a client's HTTP request lacks a critical header: lookupHeader :: RequestHeaders -> HeaderName -> Text lookupHeader hs h = decodeUtf8 $ fromMaybe notFound (lookup h hs) where notFound = throw $ MissingHeader $ decodeUtf8 $ original h The problem I am running in to is that the header isn't actually looked up until I call the AMQP library's publishMsg function. If I purposely do not supply a critical header, I get the following error printed to the screen: ConnectionClosedException "ConnectionClosedException \"UNEXPECTED_FRAME - expected content header for class 60, got non content header frame instead\"" If I add the following line just above the publishMsg function (to force evaluation), my exception handler is called successfully: print $ encode req As a result, I suspect that this is due to the fact that the "throw MissingHeader" is getting captured by the AMQP library. What's the best way to deal with this situation? -- Alex

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why is this type wrong
by Roelof Wobben 12 May '15

12 May '15

6 13

Reading, Processing, and Writing I/O
by Dananji Liyanage 12 May '15

12 May '15

Hi All, As a newcomer to Haskell, I'm following a tutorial to learn I/O in Haskell. As an exercise I'm writing a small code to read from an input text file and process it and then write it back to an output file. The problem is that, it doesn't write to the output file after the input is being processed. Here's my code: module Main where import System.IO main = do input <- readFile "input.txt" let reversedInput = reverseInput (convert input) writeFile "reversed.txt" (reversedInput) -- These two lines are not printed on the shell putStrLn (show reversedInput) putStrLn "Writing to file...." -- Reversing the input reverseInput input = (last input) : reverseInput (init input) -- Convert from String IO -> String convert x = show (x) -- Regards, Dananji Liyanage

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help to learn recursion
by Roelof Wobben 12 May '15

12 May '15

I want to learn Haskell but still have some problems on using recursion. On simple tasks I can make it work but for example the tower of Hanoi I cannot figure out how things are can be solved. Is there a book or tutorial with exercises so I can pratice it Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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Creating modules
by Dananji Liyanage 11 May '15

11 May '15

Hi All, I'm following this lecture[1] series and stuck at the module creation section. Even though I understand the idea of the module I can't figure out the exact syntax to module creation. Do we have to compile the Haskell file containing the module declaration? For an example: Do I have to compile this; module metricCalc(.....) where ... in a file metricCalc.hs [1] http://shuklan.com/haskell/index.html -- Regards, Dananji Liyanage

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Re: [Haskell-beginners] Implementation question about directed acyclic graph
by Doug McIlroy 10 May '15

10 May '15

Your suggested this representation > type Connections = M.Map Int a [Int] > > data Graph a = Empty > | Gragh { nodes :: M.Map Int (Node a), nextID :: Int, > connections :: Connections } Making connections by name (in this case an Int) entails table lookup at every step of any graph traversal. That is avoided by this simple representation: data Graph a = Graph a [Graph a] perhaps elaborated to allow the empty case data Graph a = Graph a [Graph a] | Empty If you need node identifiers, e.g. to recognize repeat visits to a node, they can be incorporated in data type a, or abstracted into another field data Eq id => Graph id a = Graph id a [Graph id a] Doug McIlroy

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Import issue
by Shishir Srivastava 09 May '15

09 May '15

Hi, I've imported the State monad module and used in the 'pop' function that will pop the head out of the list but am getting error in GHCi. ----- import Control.Monad.State let pop = State $ \(x:xs) -> (x,xs) <interactive>:73:11: Not in scope: data constructor ‘State’ Perhaps you meant ‘StateT’ (imported from Control.Monad.State) ----- Please can anyone point out what the issue is here? Thanks, Shishir

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Implementation question about directed acyclic graph
by Christian Sperandio 09 May '15

09 May '15

Hi, I’m learning the Bayesian networks and I want to write my own implementation. So, I started implementing t a DAG in Haskell but I’ve got some questions about the best way to do it in a functional mind. I’m thinking about a DAG implementation avoids duplicate information. So, I imagined this implementation: data Node a = Node { event :: a, proved :: Bool, nodeID :: Int } type Connections = M.Map Int [Int] data Graph a = Empty | Gragh { nodes :: M.Map Int (Node a), nextID :: Int, connections :: Connections } My idea is to yield an ID for each node and use this ID for connections. Thus, I can have the same events connected many times without duplicate the event data. And when I have to change a node state, I don’t need to search all occurrences in the graph. Am I on the right way ? Chris

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Uses of record syntax
by Dananji Liyanage 08 May '15

08 May '15

Hi All, I'm Dananji, a new comer to this language and functional programming as well. Can anyone please help me understand what is record syntax and its use? -- Regards, Dananji Liyanage

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